Pyrotechnic material

ABSTRACT

An infrared emitting pyrotechnic material comprising a fibrous carbon containing substrate (1) onto one or both faces (4, 5) of which is vapor deposited a combustible material layer (2, 3) which may be protected by an additional coating (6, 7). The thickness and composition of each of the layers (2, 3) are selected such that in use each of the layers is capable of igniting substantially simultaneously the entire surface on which it is deposited.

The present invention relates to a pyrotechnic material and inparticular to a pyrotechnic material suitable for use as an infra red(IR) radiation source.

Known material, such as that disclosed in U.S. Pat. No. 4,624,186,comprises thin supports, for example metal foil or paper, on to which ispressed an incendiary paste to form IR emitting flakes. The incendiarypaste is constituted with more or less incendiary material in order tospeed up or slow down its burn rate and hence control the IR emissioncharacteristics of the flakes. Here it is the paste which, in the main,acts as the IR radiation source. This has the disadvantage that becausethe pressing process used to coat the thin supports is not accuratelycontrollable the IR emission characteristics of the material so producedis not accurately controllable or reproducible.

It is an aim of the present invention to provide a pyrotechnic materialsuitable for use as an IR emitter having controllable and reproducibleIR emission characteristics.

According to the present invention there is provided a pyrotechnicmaterial characterised in that a fibrous, carbon containing substratehas vapour deposited on substantially all of the surface of one or bothfaces thereof a combustible material layer, the layer being capable inuse of igniting substantially simultaneously the entire surface on whichit is deposited.

In use this flash ignition of the surface of the carbon containingsubstrate by the combustible layer exposes a burning surface of thesubstrate which then continues to burn to act as a IR radiation source.

The duration of burning of the substrate and hence the emissioncharacteristics, such as wavelength and intensity distributions, of theIR radiation can be controlled to some extent by regulating the carboncontent of the substrate. Clearly it is essential that the substrate ofthe current invention remains for a period of time after the consumptionof the combustible layer and it has been found that in order to achievethis the carbon content of the substrate must lie in the range ofbetween 20 g/m² and 400 g/m² and should preferably lie in the range ofbetween 50 g/m² and 150 g/m². Suitable substrates may comprise aconsolidated layer of fibres, for example as in a felt or a woven carboncloth such as a carbonised rayon textile. Moreover the high degree ofcontrol over the physical characteristics of the combustible layeroffered by vapour deposition enables the emission properties of thepyrotechnic material to be reliably reproduced.

A further advantage of vapour deposition is that the combustiblematerial layer is deposited directly onto individual, exposed fibres ofthe substrate which contain, or are covered with, carbon. This maximisesthe intermingling of the carbon content of the substrate and thecombustible material layer at the interface to provide a large, intimatecontact area between the two. The resulting pyrotechnic materialexhibits considerable resistance to spontaneous ignition but, largelybecause of this intimate contact, the controlled ignition of thecombustible layer at any selected location spreads substantiallysimultaneously across the entire layer. Intimate interfacial contact,and consequentially the ignition transfer through the combustible layer,is further enhanced by the nature of vapour deposition processes whichare conventionally conducted in essentially oxygen-free environmentssuch as a vacuum or a low pressure inert atmosphere, so preventing anyinhibiting film of oxide which may form between the combustible materiallayer and the carbon containing substrate. Furthermore, vapourdeposition ensures that the advantageous properties of the textile typesubstrate base material (such as flexibility, strength, and toughness)are not substantially degraded during the manufacture of the pyrotechnicproduct.

The thickness and composition of the combustible material layer isselected to ensure reliable and rapid progression of the ignitionthrough the combustible material layer and to generate sufficient energyto establish combustion of the substrate surface. If the layer is toothick then excessive heat conduction from the interface into thecombustible material layer itself may occur and consequently thereaction may self progress too slowly to provide the required rapidignition of the substrate. Whereas if too thin then insufficient heatwill be generated by the combustion of the layer to ignite thesubstrate. For these reasons the combustible material layer thicknessdeposited on one or both faces of the substrate should be between 5microns and 200 microns per face and most preferably between 20 micronsand 80 microns per face. Since the substrate is both porous andcompressible then measurement of the thickness of any layer actuallydeposited onto the substrate may be inaccurate. The layer thicknessesquoted herein are therefore actually the thickness of layerscontemporaneously deposited onto a non-porous reference substrate, forexample an adhesive tape, placed within the deposition chamber proximalto the fibrous, carbon containing substrate.

Combustible metallic materials are particularly suitable for use as thecombustible material layer since when deposited using a vapourdeposition process the metallic materials form a highly porous layer.This porous layer provides a greatly enhanced surface area over whichthe oxidation reaction can occur and so facilitates the rapid spread ofignition through the combustible layer.

The combustible metallic layer may comprise a single metal, two or moremetals deposited either as separate layers as an alloy or as anintermetallic or any combination of individual alloy/metal/intermetalliclayers. Alternatively, thermite type multi-layers maybe used whichcomprise alternate layers of metal and metal oxide, the oxide beingformed by regulating oxygen fed into the reaction chamber of a vapourdeposition system, and may for example consist of alternating layers ofaluminium and iron oxide.

Irrespective of how the metallic material combustible layer isconstituted the selected metal is preferably one which reacts rapidly inair to generate sufficient heat when ignited to initiate the burning ofthe carbon containing substrate. Because of this and its readyavailability, it is particularly preferred that the combustible layercomprises magnesium. The metallic material layer may comprise analternative metal or an alloy thereof, particularly metals known toreact vigorously with air, such as aluminium, boron, beryllium, calcium,strontium, barium, sodium, lithium and zirconium. A layer of magnesiumor magnesium alloy of between 40 microns and 60 microns thick per face,is especially preferred, for example deposited on to one or both facesof a carbonised viscose rayon textile.

In order to extend the storage life of such a pyrotechnic material andto stabilise the ignition properties of the combustible material layer aprotective layer may be deposited on top of the combustible materiallayer. This protective coating may suitably consist of a vapourdeposited layer of a less reactive metal, for example titanium oraluminium (in cases where a more easily combustible metal is used, forexample magnesium), of between 0.1 microns and 10 microns thick andpreferably no more than 1 micron thick or may consist of a non-metalliccoating deposited onto the combustible material layer using conventionalspray or dip deposition techniques.

Most usefully the pyrotechnic material may additionally comprise anoxidant deposited onto the substrate. This oxidant provides a source ofoxygen which is available to enhance the speed of ignition transferthrough the combustible layer; to enable the substrate to continue toburn in conditions where the atmospheric oxygen is limited (for exampleif the material is used inside a closed container); and to control, tosome extent, the burn time and hence the IR emission characteristics ofthe substrate.

Where the substrate comprises a consolidated layer of fibres, such as ina carbon cloth, which is able to absorb liquid then it is convenient todeposit the oxidant onto the substrate in solution. Suitable oxidantsare water soluble inorganic salts such as metal nitrates, nitrites,chlorates and perchlorates. For example where carbon cloth is passedthrough a 5% w/w aqueous solution of potassium nitrate its burn time isincreased but if passed through a 5% w/w aqueous solution of potassiumphosphate its burn time is reduced.

It will be appreciated by those skilled in the art that an oxidantcontaining substrate may also be achieved using a suitable pre-treatmentfor the carbon containing textile, for example the introduction of leadacetate and copper during the carbonisation process of the substratematerial leads to a fibrous activated carbon substrate having lead oxideas an oxidant, without the need to separately deposit an oxidant.

An embodiment of the pyrotechnic material according to the presentinvention together with a use for this material will now be described byway of example only with reference to the accompanying drawings inwhich:

FIG. 1 shows a part sectioned view of the pyrotechnic material.

FIG. 2 shows an electron micrograph of an exposed carbon fibre of thepyrotechnic material of FIG. 1.

FIG. 3 shows the relative intensity variation in the total IR radiationoutput of the material of FIG. 1 with time.

Referring now to FIG. 1, the pyrotechnic material consists of acarbonised viscose rayon substrate 1 having combustible layers 2,3 eachconsisting of approximately 40 microns thick magnesium, vapour depositedonto substantially all of the surface of the respective faces 4,5thereof. Further layers 6,7 of titanium as a protective coat are vapourdeposited to a thickness of approximately 0.5 microns onto the exposedsurfaces 8,9 of the combustible layers 2,3.

The substrate 1 is formed from a 2.5 cm×10 cm×150 micron, 110 g/m² fibrecontaining viscose rayon tape. The tape is then carbonised in thepresence of a copper salt activating agent and a potassium salt oxidantprecursor at around 1200° C. using a conventional pyrolysiscarbonisation process comprising four stages: precarbonisation, wherephysically adsorbed solvents, water or monomers are removed;carbonisation (between 300 and 500° C.), during which oxygen, nitrogenand halogens are removed and conjugation and crosslinking occurs betweenthe carbon units; dehydrogenation (between 500 to 1200° C.), increasingthe interconnection of the conjugated carbon; and annealing (above 1200°C.) where the material attains a more crystalline structure and defectsare gradually removed. The substrate 1 so formed is highly porous andhas lead oxide as an oxidant absorbed therein.

The layers 2,3,6,7 are deposited using conventional vacuum depositionequipment (not shown). The deposition source material may be located ina separate vaporising boat (not shown) and vaporised either by heatingthe boat or by scanning the surface of the deposition source with anelectron beam in an inert atmosphere such as argon gas. Alternatively,the source may comprise a bar of material which is subjected tomagnetron sputtering or inductive coil evaporation.

The magnesium is deposited directly onto the exposed surface of thesubstrate 1 to form the combustible material layers 2,3. FIG. 2 is anelectron micrograph at ×2000 magnification showing an exposed carbonisedfibre 10 at the surface of the substrate having a radial deposit 11 of 5microns of magnesium.

The pyrotechnic material thus fabricated may be edge-trimmed prior touse to remove any uncoated substrate 1.

The typical variation in the intensity of the total radiation emissionof the material shown in FIG. 1 with time is represented in FIG. 3.

I claim:
 1. A pyrotechnic material characterised in that a fibrous, carbon containing substrate has vapour deposited on substantially all of the surface of one or both faces thereof a combustible material layer, the layer being capable in use of igniting substantially simultaneously the entire surface on which it is deposited.
 2. A pyrotechnic material as claimed in claim 1 characterised in that the carbon content of the substrate is between 20 g/m² and 400 g/m².
 3. A pyrotechnic material as claimed in claim 2 characterised in that the carbon content of the substrate is between 50 g/m² and 150 g/m².
 4. A pyrotechnic material as claimed in claim 1 characterised in that the substrate comprises a consolidated layer of fibres.
 5. A pyrotechnic material as claimed in claim 4 characterised in that the substrate is formed from a woven carbon cloth.
 6. A pyrotechnic material as claimed in claim 5 characterised in that the woven carbon cloth is a carbonised rayon textile.
 7. A pyrotechnic material as claimed in claim 1 characterised in that combustible material layer is between 5 microns and 200 microns thick.
 8. A pyrotechnic material as claimed in claim 7 characterised in that the combustible material layer is between 20 microns and 80 microns thick.
 9. A pyrotechnic material as claimed in claim 1 characterised in that the combustible material layer comprises a combustible metallic material having metals selected from the group magnesium, aluminium, boron, beryllium, calcium, strontium, barium, sodium, lithium and zirconium.
 10. A pyrotechnic material as claimed in claim 9 characterised in that the combustible layer comprises a layer of magnesium of between 40 microns and 60 microns thick.
 11. A pyrotechnic material as claimed in claim 9 further comprising a layer of a less reactive metal vapour deposited onto the exposed surface of the combustible material layer.
 12. A pyrotechnic material as claimed in claim 11 characterised in that the layer of a less reactive metal consists of a layer of titanium or aluminium of between 0.1 microns and 10 microns thick.
 13. A pyrotechnic material as claimed in claim 11 characterised in that the thickness of the less reactive metal layer is no greater than 1 micron.
 14. A pyrotechnic material as claimed in claim 1 characterised in that the material further comprises an oxidant deposited onto the substrate.
 15. A pyrotechnic material as claimed in claim 14 characterised in that the oxidant is a water soluble inorganic salt. 